A conventional engine control device employs a throttle actuator for generating to operate a throttle valve through an electrical signal for controlling the amount of intake air sucked into a vehicular engine. Specifically, the pressure of intake air sucked into the engine is sensed by a pressure sensor, and the width of pulses for driving a fuel injector disposed in an intake passage or manifold is controlled in accordance with the pressure value thus sensed so that the injector is driven every one or two engine revolutions in synchronization with the output signal of an engine rotation sensor which picks up the number of revolutions per minute of the engine. In this manner, the pulse width for the fuel injector is determined to match the intake air pressure so that a desired amount of fuel is supplied to the engine. Such control of fuel supply to the engine has been widely used as a speed-density type control and hence a further detailed description thereof will be unnecessary.
The amount of intake air sucked into an engine is controlled by a throttle valve which is disposed in the intake passage and which is in general mechanically opened and closed by a driver through a cable connected between the throttle valve and an accelerator pedal. Recently, however, it was proposed in Japanese Patent Application Laid-Open No. 61-126346 that, instead of directly connecting a throttle valve with an accelerator pedal through a cable, the throttle valve be electrically actuated by an electric actuator, and a portion of such an engine control device has been reduced to practice.
The conventional engine control device described above operates as shown in the flow chart of FIG. 1. Specifically, in Step 10, the output of the accelerator pedal sensor representative of the amount of operation .alpha. of an accelerator pedal imparted by the driver of a vehicle is read out, and in Step 11, the number Ne of revolutions per minute of the engine (hereinafter abbreviated as RPM) sensed by the engine rotation sensor and the pressure Pb of intake air are read out. Then, in Step 12, a target degree .theta. of opening of the throttle valve is calculated based on at least one of .alpha., Ne and Pb thus read out. In general, the target degree .theta. of throttle opening corresponds basically to the amount of accelerator pedal operation o modified or corrected, as necessary, by engine RPM Ne and intake air pressure Pb. For example, in a range in which the engine RPM Ne is low, the rate of change in the amount of intake air greatly changes with slight changes in the throttle opening degree. Hence it is rather difficult for the driver to precisely control the amount of intake air to be sucked into the engine by adjusting the amount of operation or depression of the accelerator pedal. To cope with this, it is proposed that in the low RPM range, the rate of change in the opening degree of the throttle valve be made smaller with respect to changes in the amount of accelerator pedal operation .alpha.. On the other hand, it has also been considered that a target value of engine RPM Ne or vehicle speed be set by the operation amount .alpha. of the accelerator pedal so that the actual throttle opening is controlled by feedback based on the difference between the target value and the, sensed value of engine RPM Ne or vehicle speed. Furthermore, since the intake pressure Pb is a physical quantity which corresponds to the output torque of the engine, it is possible to improve driving comfort by properly adjusting the throttle opening based on the difference between a sensed actual value of intake pressure and a target value which is preset based on the operation amount .alpha. of the accelerator pedal. Accordingly, in Step 13, the throttle actuator is driven by an instruction of the control unit to control the throttle valve in such a manner that the actual throttle opening is made to be the target value .theta.. In this case, the throttle actuator may be a pulse-driven open-loop control type actuator such as a stepping motor or a position-feedback control type actuator such as a DC motor.
FIG. 2 illustrates a timing chart of the conventional engine control device described above. From this chart, it will be seen that the throttle opening rapidly increases as the amount .alpha. of accelerator pedal operation or depression rapidly increases.
With the above-described conventional engine control device, when the operation amount of the accelerator pedal increases swiftly, the output torque of the engine increases sharply so that jerk or change in rate of acceleration of the vehicle in which such an engine is installed becomes greater. Accordingly, the vehicle can have excellent acceleration performance, but the riding comfort thereof is impaired. This is because reactive force, which develops upon rapid acceleration of the vehicle and is transmitted through the engine mounts to the vehicle body due to the general construction of the vehicle, causes the vehicle body to vibrate and at the same time pitching or surging thereof will be induced through the suspension system of the vehicle. In particular, the greater the jerk of the vehicle, the greater discomfort or uneasiness the driver feels.